Zeolites have remarkable structural and chemical versatility, arising from their wide range of framework* configurations, the variable populations of extra-framework species occupying their structural voids, and their ability to adsorb molecular species. These characteristics underpin unique properties like cation exchange, molecular sieving, selective adsorption, and catalytic behavior. Their framework flexibility allows dynamic responses to environmental stimuli, including temperature, pressure, and relative humidity, leading to phenomena such as negative thermal expansion (contraction), amorphization, and dehydration. Under pressure, zeolites can undergo pressure-induced incorporation of ions and molecules from surrounding fluids, modifying their structure and properties. Understanding these responses opens new pathways for tailoring zeolite behavior for technological applications and deepening our comprehension of their role in geological processes.
Rocks containing natural zeolites have been used since antiquity as components of concrete and mortars*. As pozzolans, natural zeolites react with slaked lime (Ca(OH)2) or Portland cement and water to form sparingly soluble calcium silicate and calcium aluminate hydrates. This pozzolanic reaction improves the long-term strength and durability of the mortar and concrete. Moreover, natural zeolites may replace Portland cement, reducing CO2 emissions from cement production, thereby making them an interesting decarbonization solution. Ongoing mineralogical investigations have revealed the influence of zeolitic framework topology, Si/Al ratio, and cation-exchange capacity on the pozzolanic reactivity of natural zeolites within cementitious matrices. This research paves the way to better utilization of this natural resource and leveraging its CO2-saving potential in construction.
The unique properties of natural zeolites have been exploited for decades in a myriad of applications. Recent innovative applications of natural zeolites have applied surfactants* to modify their surfaces, augmenting their normal cation-exchange property with an ability to interact with non-polar and anionic species. These surfactant-modified zeolites have been studied for removal of viruses, bacteria, pharmaceuticals, organic compounds, and oxyanions (including AsO43−) from waters and of mycotoxins from animal feed. They have also been applied as active ingredients in medical devices, drug carriers, and wound-healing accelerators. Natural zeolites have been evaluated in wine and beer making, often producing superior results when combined with bentonite. Applications for natural zeolites continue to grow, but the polymineralic nature of most deposits must be considered.
Natural zeolites typically form in low-temperature (<200 °C) environments under alkaline conditions (pH > 8). Their formation generally follows a thermal–chemical gradient, which often results in a zonation pattern of zeolites and other alteration minerals that can be linked to the physicochemical conditions existing during formation. Careful investigation of mineral compositions, sequences, and patterns at different scales facilitates unraveling past fluid–rock interactions. Investigations of the time-dependent alteration and formation of zeolites in young volcanic tephra deposits allow for a better understanding of the variability of zeolite mineralogy with respect to the effects of temperature, fluid chemistry, hydrological conditions, and time.
Zeolites commonly occur as microscopic crystals and are often overlooked. However, we benefit from these minerals in many unrecognized ways. Their distinctive crystal structures result in their ability to hydrate/dehydrate reversibly, to exchange cations with aqueous solutions, and to act as molecular sieves*. These structure-related physicochemical properties make zeolites an important group of industrial minerals. In nature, they form as one of the first alteration products of continental and oceanic crust under low-temperature conditions. The distribution and occurrence of zeolites can provide important constraints on the spatial and temporal evolution of geological systems within the shallow crust.
